Multi-element electromechanical actuation mechanism for asymmetric optical applications

ABSTRACT

Techniques for adjusting an optical element using an actuator assembly. The optical element is adjusted by translating the optical element along a linear axis of motion or by applying force upon a surface of the optical element. The optical element can be a liquid lens that is shaped by the application of force upon a surface of the lens. The actuator assembly includes a plurality of lead screws and a mechanical linkage that intercouples the lead screws and that is configured to simultaneously rotate the plurality of lead screws. The actuator assembly further includes a displacement element configured to act upon the optical element, through translational motion of the displacement element in response to rotation of the lead screws. Multiple optical elements can be adjusted simultaneously using respective displacement elements coupled to the lead screws.

BACKGROUND

Some systems have the ability to move and reposition optical elements.For example, uniaxial lens translation mechanisms exist for variousconsumer products, scientific instruments, medical devices, and sensingsystems. These mechanisms are generally employed in applications such assensor imaging for zoom and autofocus functions, as well as in laserbeam shaping and divergence control. Cameras, lasers, and sensor opticalsystems often use symmetrical cylindrically-profiled lens elements thatare displaced using a single linear or rotary actuator to achieve adesired uniaxial motion. Another adjustment mechanism employs a “tubewithin a tube” design comprising a motor powered rotating collar havinghelical slots that displace an internal lens holder. The lens holder isdisplaced using cam followers that engage with the helical slots.

A problem arises in the case when the optical element is asymmetric(e.g., a non-circular lens) and/or requires unequal, precise,displacements at several points around the perimeter of the opticalelement. In addition, low size, weight, power, and cost are desirablefor the optical path components, especially when the optical element isused in a wearable device such as head-mounted display (HMD). HMDs are awearable form of near-eye display (NED) and are sometimes used fordisplaying content in an augmented reality (AR) or virtual reality (VR)system. In the case of an HMD, a user typically views displayed contentthrough an optical aperture, which should be kept free of obstructionsthat might block the view of the user.

SUMMARY

Described herein is an actuator assembly comprising an electromechanicalactuation mechanism for adjusting an optical element in an opticalsystem. In some embodiments, the optical element is adjusted bytranslating the optical element along a linear axis of motion, movingthe entire optical element. In other embodiments, the optical element isadjusted by applying force upon a surface of the optical element. Forexample, in some embodiments, force is applied upon a flexible membraneof a liquid lens to shape the liquid lens. Embodiments described hereinare suitable for use with symmetric (e.g., circular) optical elements,but are especially advantageous for applications that involve asymmetricoptical elements, as well as applications that require non-uniform(unequal) force or displacement around a perimeter of an optical elementbeing adjusted. In particular, some embodiments may be used for applyingunequal force around the perimeter of a non-circular lens in order toachieve a desired optical effect (e.g., a desired optical power), and tosynchronously apply non-uniform force/displacement to multiple lenses.

In certain embodiments, an actuator assembly includes a plurality oflead screws and a mechanical linkage that intercouples the plurality ofleads screws. The mechanical linkage is configured to simultaneouslyrotate the plurality of lead screws. The actuator assembly furtherincludes at least one displacement element. Each displacement element isconfigured to act upon a respective optical element to which thedisplacement element is coupled, through translational motion of thedisplacement element in response to rotation of the plurality of leadscrews.

In certain embodiments, a system includes a head-mounted device and anactuator assembly. The head-mounted device includes an optical systemwith at least one optical element, wherein at the least one of theoptical element includes a lens. The actuator assembly is housed withinthe head-mounted device and includes a plurality of lead screws and amechanical linkage that intercouples the plurality of leads screws. Themechanical linkage is configured to simultaneously rotate the pluralityof lead screws. The actuator assembly further includes at least onedisplacement element. Each displacement element is configured to actupon a respective optical element of the at least one optical element,through translational motion of the displacement element in response torotation of the plurality of lead screws.

In certain embodiments, a method includes determining, by one or moreprocessors of a computer system, a desired optical characteristic of anoptical system including at least one optical element. The methodfurther includes determining, by the one or more processors, a targetposition for at least one displacement element in an actuator assemblybased on the desired optical characteristic. The actuator assemblyincludes an actuator configured to produce rotational output, aplurality of lead screws, and a mechanical linkage that intercouples theplurality of leads screws. The mechanical linkage is configured tosimultaneously rotate the plurality of lead screws based on therotational output produced by the actuator. The actuator assemblyfurther includes the at least one displacement element. Eachdisplacement element is configured to act upon a respective opticalelement of the at least one optical element, through translationalmotion of the displacement element in response to rotation of theplurality of lead screws. The method further includes causing, by theone or more processors, power to be applied to the actuator to move theat least one displacement element toward the target position.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments are described with reference to the followingfigures.

FIGS. 1 and 2 show examples of near-eye displays suitable forimplementing one or more embodiments.

FIG. 3 shows a cross section of a near-eye display suitable forimplementing one or more embodiments.

FIG. 4 is a perspective view of an actuator assembly, according to anembodiment.

FIG. 5 is a front view of the actuator assembly of FIG. 4.

FIG. 6 is an exploded view of the actuator assembly of FIG. 4.

FIG. 7 is a perspective view of a floating drive nut mechanism that canbe used to implement an actuator assembly, according to an embodiment.

FIG. 8 is a front view of a belt slip prevention mechanism that can beused to implement an actuator assembly, according to an embodiment.

FIG. 9 shows cross-sectional views of an actuator assembly in differentstates of actuation, according to an embodiment.

FIG. 10 is a flowchart of a method for adjusting an optical system usingan actuator assembly, according to an embodiment.

FIG. 11 is a block diagram of a system in which one or more embodimentsmay be implemented.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated may be employed without departing from theprinciples, or benefits touted, of this disclosure.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, specificdetails are set forth in order to provide a thorough understanding ofcertain inventive embodiments. However, it will be apparent that variousembodiments may be practiced without these specific details. The figuresand description are not intended to be restrictive.

Example embodiments relate to an electromechanical actuation mechanismfor adjusting an optical element in an optical system, for example, bydisplacing all or a portion of the optical element. In some embodiments,the optical element is adjusted by translating the optical element alonga linear axis of motion, moving the entire optical element. In otherembodiments, the optical element is adjusted by applying force upon asurface of the optical element. For example, in some embodiments, forceis applied upon a flexible membrane of a liquid lens to shape the liquidlens. Embodiments described herein are suitable for use with symmetric(e.g., circular) optical elements, but are especially advantageous forapplications that involve asymmetric optical elements, as well asapplications that require non-uniform (unequal) force or displacementaround a perimeter of an optical element being adjusted. In particular,some embodiments may be used for applying unequal force around theperimeter of a non-circular lens in order to achieve a desired opticaleffect (e.g., a desired optical power), and to synchronously applynon-uniform force/displacement to multiple lenses.

Example applications for an embodiment of the present disclosure includemoving or distorting non-circular ophthalmic lenses for presbyopiacorrection or visual accommodation correction in a stand-alone HMD(e.g., an AR or VR device with an integrated controller) or in a systememploying an HMD (e.g., an AR/VR device and a remote console controllingthe AR/VR device). Accommodation refers to the change of optical powerwithin the human eye as distance to the viewed object changes.Presbyopia is an age related condition characterized by lack of range offocus of the eye and inability to focus on close objects. An embodimentof a system according to the present disclosure could performaccommodation correction to provide a better viewing experience to theuser by, for example, changing the relationship of the focal distance ofa display image to a real-world image to correct for the natural focalshift of the user's eye when looking at near field objects, therebykeeping the display image in focus regardless of the eye's natural focalplane. The system could also correct for presbyopia by, for example,adaptively changing a focal length of a lens to provide the necessarycorrective optical power to the user's eye at different focal distances(e.g., continuously adjusting the focal length to cover all near fieldfocal distances).

Other potential applications include manipulation of non-circular lensesthat are not used for viewing by a user, such as lenses in a sensorsystem. Embodiments can also be applied for asymmetric laser beamshaping or any other application where displacement or distortion of anoptical element is desired.

In some embodiments, the optical elements include one or more liquidlenses. However, it is understood that the embodiments can be appliedfor displacement/distortion of other types of optical elements, such assolid lenses. Liquid lenses comprise a sealed cavity filled with fluid,e.g., a fluid having one or more desired properties such as a particularindex of refraction, a particular viscosity, and/or a particular degreeof light transmissivity. According to some embodiments, a liquid lens isshaped by expanding or compressing a flexible membrane of the lens.Expansion or compression of the membrane causes the fluid to bedisplaced to create an optical surface on one side of the lens. Forexample, optical power can be changed by applying pressure to themembrane to mechanically displace fluid from the perimeter of the lenstoward the optical center of the lens, causing the membrane to bulge,thereby increasing refraction of light and thus the optical power of thelens.

Example embodiments relate to an electromechanical actuation mechanismthat is operable to effect displacement of an optical element atmultiple points around a perimeter of the optical element, usingcomponents located outside of an optical aperture. In this manner, theactuation mechanism can precisely control the displacement withoutimpeding light transmission or image quality through the opticalelement, making the actuation mechanism especially suited for use withHMDs and other wearable devices where the optical aperture is a viewingaperture.

Example embodiments relate to an actuator assembly comprising anelectromechanical actuation mechanism for synchronized displacement of aplurality of optical elements that are intercoupled to permit theoptical elements to be driven by a single actuator. The ability toprecisely control displacement of multiple optical elements using asingle actuator facilitates flexible and rapid configuration of theoptical system while minimizing size, weight, power consumption, andcost.

In some embodiments, an actuator assembly includes an actuator operableto produce rotational output (e.g., a motor), a plurality of lead screwsincluding a first lead screw driven by the rotational output of theactuator, and a mechanical linkage (e.g., a belt or cable) configured tosimultaneously rotate the plurality of lead screws (e.g., bydistributing torque from the first lead screw to a remainder of theplurality of lead screws). The actuator assembly further includes atleast one displacement element (e.g., a lens holder or displacementring). Each displacement element is configured to act upon a respectiveoptical element to which the displacement element is coupled, throughtranslational motion of the displacement element in response to rotationof the plurality of lead screws.

In some embodiments, a system includes an HMD that houses an actuatorassembly. The HMD includes an optical system with a plurality of opticalelements, at least one of which is a lens that the actuator assemblyacts upon.

In some embodiments, a method performed by one or more processors of acomputer system includes determining, by the one or more processors, adesired optical characteristic of an optical system including at leastone optical element. The method further includes determining (e.g.,mapping or calculating), by the one or more processors, a targetposition for at least one displacement element in an actuator assemblybased on the desired optical characteristic. The method further includescausing, by the one or more processors, power to be applied to anactuator of the actuator assembly to move the at least one displacementelement toward the target position (e.g., using a belt or cable tosimultaneously drive a plurality of lead screws).

In some embodiments, a displacement element is moved by differentamounts (i.e., non-uniform displacement) at various points along thedisplacement element. In some embodiments, different displacementelements are non-uniformly displaced with respect to each other. Themovements for a single displacement element or for multiple displacementelements can be synchronized using a mechanical linkage that causes aplurality of lead screws to simultaneously rotate. Depending on how thelead screws are threaded, different amounts and/or directions ofdisplacement can be produced.

Embodiments of the present disclosure may include or be implemented inconjunction with an artificial reality system. Artificial reality is aform of reality that has been adjusted in some manner beforepresentation to a user, which may include, e.g., a VR, an AR, a mixedreality (MR), a hybrid reality, or some combination and/or derivativesthereof. Artificial reality content may include completely generatedcontent or generated content combined with captured (e.g., real-world)content. The artificial reality content may include video, audio, hapticfeedback, or some combination thereof, and any of which may be presentedin a single channel or in multiple channels (such as stereo video thatproduces a three-dimensional effect to the viewer). Additionally, insome embodiments, artificial reality may also be associated withapplications, products, accessories, services, or some combinationthereof, that are used to, e.g., create content in an artificial realityand/or are otherwise used in (e.g., perform activities in) an artificialreality. The artificial reality system that provides the artificialreality content may be implemented on various platforms, including anNED connected to a host computer system, a standalone NED, a mobiledevice or computing system, or any other hardware platform capable ofproviding artificial reality content to one or more viewers.

FIG. 1 shows an NED 100 suitable for implementing one or moreembodiments. The NED 100 presents media to a user. Examples of mediapresented by the NED 100 include one or more images, video, and/oraudio. In some embodiments, audio is presented via an external device(e.g., speakers and/or headphones) that receives audio information fromthe NED 100, a console, or both, and presents audio output based on theaudio information. The NED 100 can be configured to operate as a VRdisplay. In some embodiments, the NED 100 is modified to operate as anAR display and/or an MR display.

The NED 100 includes a frame 105 and a display device 110. The frame 105is shaped to enable the NED 100 to be worn in the manner of a pair ofeyeglasses. Thus, the NED 100 is an example of an HMD. The frame 105 iscoupled to one or more optical elements (e.g., lenses integral with thedisplay device 110). The display device 110 is configured for the userto see content presented by NED 100. In some embodiments, the displaydevice 110 comprises a waveguide display assembly for directing lightfrom one or more images to an eye of the user.

The NED 100 may include one or more optical sensors (not shown) thatcapture optical data about the user and/or the external environment. Forexample, the optical sensors may include at least one pixel cell arraycomprising an plurality of pixel cells (e.g., a two-dimensional (2D)pixel cell array) configured to generate image data representing aparticular field of view along a particular direction toward the user ortoward the external environment.

In some embodiments, the NED 100 may include one or more activeilluminators configured to project light toward the user and/or towardthe external environment. Active illuminators are activated usingelectrical signals that cause the illuminators to project light. Theprojected light may form one or more light patterns, can be associatedwith different frequency spectrums (e.g., visible light, infrared (IR)light, near infrared (NIR) light, ultra-violet (UV) light, etc.), andcan serve various purposes, including illuminating the user's face inconnection with eye tracking or facial recognition and illuminating theexternal environment in connection with tracking of the location or headmovement of the user.

FIG. 2 shows an NED 200 suitable for implementing one or moreembodiments. Like the NED 100, the NED 200 is an HMD designed to be wornby a user. The NED 200 includes a housing 220 and a display device (notshown) inside the housing. The display device may be positioned near afront side 225 of the NED 200. The housing 220 forms an enclosed viewingenvironment for the user and includes a pair of eye cups 230-A and 230-Bthat are attached to a top side 223 of the NED and surround the eyes ofa user when the NED is being worn. The NED 200 further includes a strap240 configured to secure the NED against a back of the user's head andvarious electronics (e.g., an antenna unit 250 for wirelesscommunication with a remote computing device) located on a right side227 of the NED 200. Both the NED 100 and the NED 200 may include opticalelements that are integral with a display device or located along anoptical path between the display device and an eye of the user.

FIG. 3 shows a cross section of an NED 300 suitable for implementing oneor more embodiments. The NED 300 may correspond to the NED 100 or theNED 200. The NED 300 includes a frame or housing 305, a display device310, and an optical system 340. The display device 310 is configured topresent image content to the user. The optical system 340 is configuredto direct image light from the display device 310 to an eye 320 of theuser. The optical system 340 may include a waveguide, lenses, and/orother optical elements that guide or adjust the image light 350. In someembodiments, additional optical elements are embedded within the displaydevice. When placed into an operative position with respect to the user,e.g., when the user wears the NED 300, the NED 300 forms an exit pupil330 at a location where the eye 320 is positioned in an eyebox region.For purposes of illustration, FIG. 3 shows the cross section associatedwith a single eye 320 and optical system 340, but a second opticalsystem 340 can be used for a second eye of the user.

The optical system 340 is configured to direct the image light to theeye 320 through the exit pupil 330. The optical system 340 may includeoptical elements composed of one or more materials (e.g., plastic,glass, etc.) with one or more refractive indices. For example, theoptical system 340 may include a waveguide composed of one or morematerials with one or more refractive indices that effectively minimizethe weight and widen a field of view (FOV) of the NED 300. The opticalsystem may include other types of optical elements that adjust or guidethe image light 350, e.g., to correct aberrations in the image light ormagnify the image light. Example optical elements include an aperturestop, a Fresnel lens, a convex lens, a concave lens, a filter, areflector, or any other suitable optical element that affects imagelight. In some embodiments, the optical system is part of or attached tothe display device 310. In other embodiments, the optical system isseparate from the display device 310 and located along an optical pathbetween the eye 320 and the display device 310. The optical system maybe driven by a controller of the NED 300 to, for example, adjust one ormore optical characteristics of the optical system 340 using anelectromechanical actuation mechanism in accordance with an embodimentdescribed herein.

FIG. 4 shows a perspective view of an actuator assembly 400 according toan embodiment. The actuator assembly 400 can be incorporated into awearable device that includes an optical system, e.g., an HMD. Theactuator assembly 400 includes an aperture 405, an actuator 410, leadscrews 420-A to 420-D, tension idlers 425-A to 425-C, displacement rings430-A and 430-B, a belt 440, a housing 450, an end plate 455, and lenses460-A and 460-B. For simplicity, other types of optical elements havebeen omitted. However, it is understood that the actuator assembly 400can include optical elements besides lenses. For example, the lenses 460may be stacked together with a removable prescription lens or a Fresnellens. Further, although described with respect to synchronizeddisplacement of a pair of optical elements (the lenses 460-A and 460-B),the actuator assembly 400 can be adapted for displacement of any numberof optical elements. For example, an additional displacement ring couldbe introduced for each optical element to be displaced. The embodimentof FIG. 4 can be used for translation of lenses or other opticalelements by moving the displacement rings 430.

The aperture 405 forms a viewing window through which the user can seethrough the lenses 460-A and 460-B. The aperture 405 can include a layerof clear or light transmissive material that forms a protective layerfor the lenses. In some embodiments, protective layers may be attachedto both the housing 450 and the end plate 455 to seal the actuatorassembly 400 against intrusion of dust, water, or other contaminants.Alternatively, the aperture 405 can be an opening defined by the housing450 and the end plate 455.

The actuator 410 can be an electrically activated motor that generatesrotational motion. Various types of motors are suitable for implementingthe actuator 410, including stepper type, servo, direct current (DC),alternating current (AC), brushed, or brushless motors. In FIG. 4, theactuator 410 is mounted on a user facing side of the housing 450. Otherlocations for the actuator 410 are equally feasible including, forexample, on the end plate 455 or along a side of the housing 450. Theactuator 410 is coupled to the lead screw 420-A through a gear setincluding a gear 412 attached to the output end of the actuator 410 anda gear 414 (shown in FIG. 6) attached to the lead screw 420-A. The gearset can be implemented using spur gears (as shown in the figures) if theactuator 410 is mounted with its axis of rotation parallel to thelongitudinal axes of the lead screws 420. Alternatively, the gear setcan be implemented using bevel gears if the actuator 410 is mounted withthe axis of rotation perpendicular to the longitudinal axes of the leadscrews 420. The ratio of the gears can be set to achieve a desiredtravel speed for the displacement rings 430. For example, a speedreduction and torque amplification can be achieved using a smaller drivegear (e.g., the gear 412) than driven gear (e.g., the gear 414).

The lead screws 420 are located around the periphery of the opticalelements, e.g., the lenses 460. The lead screws are placed outside ofthe aperture 405 in order not to obstruct the view of the user. The leadscrews 420 operate to mechanically support the displacement rings 430and to move the displacement rings 430 linearly in response torotational output of the actuator 410. Each lead screw includes athreaded shaft in contact with the displacement rings 430 and a toothedhead in contact with the belt 440. The shaft and head are shown moreclearly in FIGS. 6 and 7. The threading of the shaft converts the rotarymotion of the actuator 410 to linear motion (in FIG. 4, motion along thez direction). The lead screws 420 can be threaded in various ways. Ifuniform displacement is desired, the threading can be made uniform. Inone embodiment, each lead screw has an individual, unique thread pitchso as to provide non-uniform displacement. For example, the distancebetween threads of the lead screw 420-A could be different from thedistance between threads of the lead screw 420-B. For simplicity, thethreading has been omitted from the figures, which depict the shaftsections of the lead screws as being smooth.

In some embodiments, one or more of the lead screws 420 are shaped astwin lead screws with a first threaded section at one end and a secondthreaded section at the opposite end, where the first threaded sectionis coupled to the displacement ring 430-A and the second threadedsection is coupled to the displacement ring 430-B. The first threadedsection and the second threaded section could be threaded in the samedirection (e.g., both right-hand threaded) or in opposite directions(e.g., one section is right-hand threaded and the other section isleft-hand threaded). Opposite threading of the sections would enable thelead screws to provide motion in opposite directions (e.g., so that thedisplacement rings 430-A and 430-B move away from each other). If thesections are threaded in the same direction, then the displacement rings430-A and 430-B would move in the same direction.

The lead screws 420 can be supported on low friction sintered sleevebearings or jeweled bearings (not shown). The bearings can be coupled toone or more ends of the lead screws to reduce friction and thus wearingas the lead screws are rotated. For example, bearings may be included atpoints where the lead screws meet the end plate 455.

The tension idlers 425 can be implemented as pulleys that rotate andprevent the belt 440 from contacting the end plate 455, thereby avoidingfriction between the belt 440 and the end plate 455. The tension idlers425 are positioned to provide sufficient tensioning of the belt 440 suchthat there is little or no slack that could otherwise cause the belt 440to come into contact with the end plate 455.

The displacement rings 430 are relatively thin (in the z direction) andnarrow in width (in the x direction). The thickness may be approximatelyof the same order of magnitude as the thickness of the lenses 460. Thedisplacement rings 430 may operate as lens holders for the lenses. Inthis respect, the width of the displacement rings can be minimized so asnot to obstruct the aperture 405, while being wide enough to securelyhold the lenses.

In some embodiments, the travel of the displacement rings 430 may belimited using one or more sensors, for example using end-of-travelmicro-switches positioned in the housing 450, such that the displacementrings 430 activate the micro-switches at a desired end of travel,thereby signaling a controller to shut off power supplied to theactuator 410. Alternatively, travel may be limited using a combinationof an electronic shaft encoder (e.g., an absolute position type orrelative counter type encoder) coupled with a micro-switch that ispositioned on the actuator 410. The encoder could track the number ofrevolutions and/or angular position of an output shaft of the actuator410 (e.g., the shaft to which the gear 412 is attached) to generate asignal indicating to the controller the actual position of thedisplacement rings 430, for example through a lookup table. Yet anotherway to limit travel would be to use a current sensor that senses acurrent of the actuator 410, together with a “bumper stop” element atthe end of travel (e.g., a linear spring or elastomer that compresseswith an increasing reaction force toward the end of travel). Thisresistance to motion would cause the actuator current to ramp up,allowing a current sensing controller to shut current off at a certainthreshold current level corresponding to the end of travel position.

The lenses 460 are asymmetrically shaped, with lens 460-A being mountedon the displacement ring 430-A and lens 460-B being mounted on thedisplacement ring 430-B. The lenses 460 may be solid lenses.Alternatively, as explained below in connection with FIG. 9, lenses canbe liquid lenses that include a flexible membrane. The lenses can bemounted to the displacement rings in various ways. For example, thelenses 460 may be mounted using an adhesive or other fixing agent. Insome embodiments, the displacement rings 430 may include a grooved innersurface into which the lenses are friction fit in a similar manner tolenses in an eyeglass frame. Other mounting techniques are alsopossible.

The belt 440 operates as a mechanical linkage that converts rotationalmotion of the lead screw 420-A into rotational motion of the lead screws420-B, 420-C, and 420-D. Like the lead screws 420, the belt 440 islocated outside of the aperture 405. The actuator 410 applies torque viathe gears 412 and 414 to the lead screw 420-A, causing the lead screw420-A to displace the displacement ring 430-A (e.g., by pushing againstthe displacement ring using a drive nut that is threaded onto the leadscrew, as shown in FIG. 7). The belt 440 distributes the torque appliedto the lead screw 420-A to the other lead screws 420-B, 420-C, and420-D. The belt 440 can be formed of a flexible material (e.g., apolymer or elastomer). The belt 440 can be spring loaded to increasebelt tension and therefore increase the angle of wrap against the leadscrews 420, thereby preventing slippage as the belt relaxes over thelife of the actuator assembly 400. In some embodiments, the belt 440 mayinclude teeth that engage the teeth of the lead screws 420. Otherlinkage mechanisms are also possible. For example, the belt 440 can bereplaced with a beaded cable, a roller chain, or a gear train.

The use of a flexible belt for torque distribution helps prevent noiseand vibration as compared with other torque distribution methods usingmore rigid components, due to the energy damping characteristics of theflexible material. The prevention of noise and vibration is desirablefor an HMD as the proximity of the user's ear and direct wavepropagation path from the device to the ear will make even slightvibrations and noise easily detectable to the user. In some embodiments,vibration is further reduced by using a soft motor mount between thehousing 450 and the actuator 410 to damp any motor imbalance,acceleration or deceleration forces, or other sources of rotatingmass-based vibration.

The housing 450 provides mechanical support for the actuator 410, thelead screws 420, the optical elements, and any other components whichmay reside within the housing 450. The housing can be formed of a rigidmaterial such as a metal, a metal alloy, or a polymer metal (i.e., apolymer-metal composite). The housing 450 is mated to the end plate 455and held in place against the end plate 455 by the lead screws 420-A to420-D. In some embodiments, the housing 450 and the end plate 455 areattached to each other using fixed screws, adhesive, a snap fit orfriction fit mechanism, or some other attachment mechanism. The housing450 may include space for mounting additional optical elements such asfixed-location lenses. If the actuator assembly 400 is incorporated intoan HMD or other wearable device, the housing 450 and the end plate 455can be integrated into the wearable device such that the actuatorassembly 400 is housed within the wearable device.

The end plate 455 provides thrust support for the lead screws 420 and,if bearings are included, may also provide support for the bearings. Theend plate 455 includes a groove that receives the belt 440, the tensionidlers 425, and the geared portions of the lead screws 420. FIG. 5 showsin further detail the arrangement of the lead screws 420, the belt 440,and the tension idlers 425 relative to the end plate 455.

FIG. 5 is a front view of the actuator assembly 400. As shown in thefigure, the end plate 455 includes an outer wall 550 and an inner wall560 that together form a groove in which the lead screws 420, the belt440, and the tension idlers 425 are received. The tension idlers 425apply enough tension to the belt 440 that the belt maintains continuouscontact with the lead screws and does not come into contact with thewalls 550 and 560 as the belt 440 moves along the lead screws. The walls550 and 560 can be continuous or broken into segments. For example, FIG.5 shows the inner wall 560 being formed of a first segment 562 thatspans a majority of the perimeter of the inner wall and a shortersegment 564 that extends along the bottom of the inner wall, with spacesbetween the segments.

The actuator assembly 400 can include a floating drive nut mechanism(shown in FIG. 7) that allows for a small amount of transverse movementof the displacement rings 430. The transverse movement accommodatesrotation of the displacement rings (e.g., rotation caused by non-uniformdisplacement at different points along the displacement rings) bypreventing side loading of the lead screws 420. The floating drive nutmechanism also prevents binding of the mechanical linkage (e.g., thebelt 440) that might otherwise occur as result of structural deflectionsof the housing 450.

FIG. 6 is an exploded view of the actuator assembly 400. FIG. 6 shows inmore detail the structure of the lead screws 420, each of whichcomprises a shaft section and a head section. For example, the leadscrew 420-D includes a shaft 610 and a head 620. As mentioned earlier,the shaft may be threaded while the head is toothed. The head 620operates as a sheave that rotates against the belt 440. Teeth of thehead grip the belt 440 so that the belt 440 moves in response torotation of the lead screw 420-A to drive the other lead screws 420-B,420-C, and 420-D. Other head shapes are also possible, such as atoothless groove with bumps, ridges, or some other surface texture tofacilitate gripping of the belt 440.

FIG. 7 is a perspective view of a floating drive nut mechanism 700 thatcan be used to implement an actuator assembly (e.g., the actuatorassembly 400). The housing, end plate, lenses, and various othercomponents have been omitted for clarity. The floating drive nutmechanism allows for some structural distortion of the housing as aresult of natural forces arising during operation of the actuatorassembly, without applying significant side loads to the lead screws420. Otherwise, the use of rigid, close tolerance components in thepresence of side loads could cause binding of the lead screws 420 orbinding of the belt 440.

The floating drive nut mechanism 700 includes a set of drive nuts 710-Aand 710-B with respective anti-rotation flats 720-A and 720-B. The drivenuts 710-A and 710-B can be threaded onto a lead screw 420. Only onelead screw 420 is shown, but corresponding drive nut mechanisms can beused for each of the lead screws. The drive nut 710-A contacts thedisplacement ring 430-A and the drive nut 710-B contacts thedisplacement ring 430-B. The anti-rotation flats 720 contact an innersurface of the housing 450 (not shown) to prevent rotation of the drivenuts 710 as the lead screw 420 is rotated.

The inset image shows the structure of the drive nut 710-B in moredetail. The drive nut 710-A can be similarly shaped. As shown in theinset, the drive nut 710-B includes a flanged section 712 and acylindrical section 714. The flanged section 712 contacts a back side ofthe displacement ring 430-B while the cylindrical section 714 isinserted through an opening of the displacement ring 430-B. The leadscrew 420 passes through the cylindrical section 714, which can beinner-threaded to match the threads of the lead screw. The flangedsection 712 of the drive nut 710-A is separated from the displacementring 430-A by a gap 730. The flanged section 712 of the drive nut 710-Bis likewise separated from the displacement ring 430-B. Further, thecylindrical section 714 of drive nut 710-B is separated from thedisplacement ring 430-B by a gap 732. The cylindrical section 714 ofdrive nut 710-A is likewise separated from the displacement ring 430-A.The gaps 730 and 732 provide diametrical clearance between the drivenuts and the displacement rings, allowing for a limited degree ofangular movement (e.g., rotation) of the displacement rings before sideloading occurs.

FIG. 8 is a front view of a belt slip prevention mechanism 800 that canbe used to implement an actuator assembly (e.g., the actuator assembly400). Belt slippage is a concern in the presence of external loads andhousing distortion, and could result in loss of synchronization betweenthe lead screws. The mechanism 800 comprises a small gap 805 between theend plate 455 and the belt 440 at a location of a lead screw 420. Asimilar gap 805 may be provided at the locations of each lead screw 420in the actuator assembly. The gap 805 is sized to be smaller than athickness 810 of the belt 440, preventing the belt 440 from slipping offthe lead screw 420 by limiting radial displacement of the belt withrespect to the lead screw. In particular, the gap 805 may prevent thebelt 440 from radially displacing enough to separate from contact withthe teeth 820 of the head of the lead screw 420.

FIG. 9 shows cross-sectional views of an actuator assembly 900 indifferent states of actuation. FIG. 9 illustrates the adjustment of aliquid lens using the actuator assembly 900, which includes componentssimilar to those described above with respect to the actuator assembly400. For example, as shown, the actuator assembly 900 includes a pair ofdisplacement rings 930-A and 930-B, a pair of lenses 960-A and 960-B, aplurality of lead screws 920-A and 920-B, and a belt 940. However,unlike in the actuator assembly 400, the displacement rings 930 do notoperate as lens holders, but are instead used to apply a force around aperimeter of the lenses 960, which are held stationary against a housing950 and an end plate 955 of the actuator assembly 900. The force neednot be applied directly along a boundary of the lenses (e.g., becausethe displacement rings may not be exactly aligned with the edges of thelenses). Therefore, applying a force around a perimeter of a lens orother optical element can include applying force near the edge of theoptical element. Each of the lenses 960 includes a flexible membrane 910that is displaced by the force applied around the perimeter of the lens.Opposite the membrane is a lens surface that contacts the housing 950 orthe end plate 955 and which may be rigid or semi-rigid. Alternatively,the lenses 960 can be mounted on stationary holders. Torque distributioncan be performed in the same manner as described with respect to theactuator assembly 400, e.g., using the belt 940 as a mechanical linkagethat intercouples the lead screws 920, with the lead screw 920-A beingdriven by the rotational output of an actuator (not shown).

The left side of FIG. 9 shows the displacement rings 930 in a firstconfiguration, with displacement ring 930-A resting against a membrane910-A of the lens 960-A and displacement ring 930-B resting against amembrane 910-B of the lens 960-B. In this configuration, the lenses 960are in a relaxed state, with the internal liquid being uniformlydistributed across the lens area. The uniform distribution of the liquidproduces a relatively flat membrane shape, and therefore low opticalpower. The membranes 910 may be configured to be in a state of tensionin the relaxed state, thereby providing a natural inward spring pressureagainst the displacement rings 930.

When the lead screws 920 are rotated, the displacement rings 930 moveaway from each other in the directions shown by the arrows in the rightside of the figure (e.g., if the lead screws 920 are twin lead screwswith opposite direction threading for the shaft sections that couple tothe displacement rings 930). The movement of the displacement rings 930applies pressure to the lens membranes 910, causing liquid to be pushedfrom a periphery of the lenses 960 toward the centers of the lenses, sothat the membranes 910 bulge toward the interior of the actuatorassembly 900 to form a more spherical shape that increases therefraction of light through the lenses. Thus, the configuration shown onthe right side of FIG. 9 has a higher optical power than the firstconfiguration on the left side.

In some embodiments, one or more displacement rings 930 are attached toa corresponding membrane, e.g., using an adhesive, so that when thedisplacement ring is moved away from the lens, the membrane becomesstretched, e.g., to form a concave optical surface that providesnegative optical power. Additionally, in some embodiments, thedisplacement rings 930 may flex in response to rotation of the leadscrews 920. Further, each displacement ring 930 may have a varyingcross-sectional thickness, with some sections being thicker and somesections being thinner, in order to facilitate non-uniform deflection ofthe displacement ring. For example, smaller cross-sections can be usedin areas where less deflection is desired, and larger cross-sections canbe used in areas where more deflection is desired.

Example embodiments of actuator assemblies have been described which usean electrically controlled actuator to produce rotational output thatcauses a plurality of intercoupled lead screws to rotate. In particular,the actuator drives a first lead screw to cause the remaining leadscrews to rotate via a mechanical linkage. In some embodiments, anactuator may be omitted so that rotation of a lead screw is performedmanually. Some embodiments may feature an actuator that drives anon-lead screw element (e.g., an elongated element with a non-threadedshaft and a toothed head) that is coupled to the lead screws via themechanical linkage to provide a rotational motion that causes rotationof the lead screws. Other modifications of the disclosed embodiments arealso possible.

FIG. 10 is a flowchart of a method 1000 for adjusting an optical systemusing an embodiment of an actuator assembly. The method 1000 can beapplied to adjust an optical system in an HMD, for example, to move ordistort (e.g., shape in a defined manner) lenses in the HMD in order toprovide presbyopia correction or visual accommodation correction. Themethod 1000 can also be applied to other types of optical systemsincluding, for example, optical systems that include optical elementsfor producing an image captured by an image sensor and optical systemsthat include optical elements for generating focused light output (e.g.,for asymmetric laser beam shaping). The method 1000 can be performed bya controller that is implemented in hardware, software, or a combinationthereof. For example, the controller may be a control unit of an HMD, orone or more processors executing computer-readable instructions on acomputer system.

At step 1010, the controller determines one or more desired opticalcharacteristics of the optical system. The optical characteristics maypertain to any number of parameters that define the performance of theoptical system, e.g., optical power, focal length, zoom factor, radiusof curvature, etc. The desired optical characteristics can be userselected or determined by the controller, e.g., based on a measurementof the refractive error of an eye of an HMD user.

At step 1020, the controller maps or calculates target positions of aplurality of displacement elements to which optical elements of theoptical system are coupled, based on the desired opticalcharacteristics. The displacement elements can be holders on which theoptical elements are mounted (e.g., displacement rings that act as lensholders) or elements that apply force to the optical elements (e.g., thedisplacement rings of FIG. 9). The controller may perform mapping byreferencing a stored lookup table or other data structure containinginformation indicating which target positions achieve the desiredoptical characteristics. Alternatively, the controller may calculate thetarget positions using the desired optical characteristics as input.

At step 1030, the controller applies power (e.g., a drive current orvoltage) to an actuator of the actuator assembly, causing the actuatorassembly to synchronously drive the displacement elements toward theirrespective target positions using a set of lead screws that areintercoupled via a mechanical linkage. The mechanical linkage can, forexample, include a belt or a cable. In some instances, a singledisplacement element may have multiple target positions. For example, itmay sometimes be desirable to use a displacement ring to apply differentamounts of force along a perimeter of a lens. This could be achieved byvarying the displacement at different areas of the displacement ring(e.g., each lead screw to which the displacement ring is coupled coulddrive the displacement ring for a different distance). As explainedearlier, the actuator assembly can accommodate such asymmetricdisplacement through appropriate design of the lead screws (e.g., usingvarying thread pitch, thread direction, and/or thread length). Thus, itmay even be possible for one lead screw to drive a displacement elementin one direction and another lead screw to drive the same displacementelement in the opposite direction.

In step 1040, the controller shuts off power to the actuator upondetecting that the displacement elements have reached their targetpositions. For example, the controller may receive a signal from asensor (e.g., a shaft encoder) that enables the controller to determinethe positions of the displacement elements. Further, as explainedearlier, the actuator assembly may include end-of-travel micro-switchesor some other mechanism that limits the travel of the displacementelements (e.g., to prevent the displacement elements from being drivenbeyond a specific range of positions). Thus, the controller can alsoshut off power upon detecting that an end-of-travel position has beenreached for one or more displacement elements (e.g., to shut off poweras soon as an end-of-travel position is detected for any displacementelement).

FIG. 11 is a block diagram of a system 1100 in which one or moreembodiments may be implemented. The system 1100 includes an HMD 1110, acontrol unit 1130, and an input/output (I/O) interface 1140. The HMD1110 includes a display device 1112, an optical system 1113, an actuatorassembly 1114, at least one proximity sensor 1116, at least oneilluminator 1118, at least one optical sensor 1120, at least oneposition sensor 1122, and an inertial measurement unit 1124.

The display device 1112 includes a display screen for presenting visualmedia, such as images and/or video, to a user. In addition to visualmedia, the HMD 1110 may include an audio output device (not shown) forpresenting audio media to the user, e.g., in conjunction with thepresentation of the visual media.

The optical system 1113 includes at least one optical element (e.g., alens, a waveguide, a reflector, etc.) that affects image light. Multipleoptical elements may be stacked together to, for example, direct andguide light from the display device 1112 toward an eye of the user.

The actuator assembly 1114 includes an actuator operable to producerotational output (e.g., a motor), a plurality of lead screws, and amechanical linkage (e.g., a belt or cable) configured to simultaneouslyrotate the plurality of lead screws. The actuator assembly 1114 furtherincludes a plurality of displacement elements (e.g., displacementrings). Each displacement element is configured to act upon a respectiveoptical element to which the displacement element is coupled, throughtranslational motion of the displacement element in response to rotationof the plurality of lead screws.

The actuator assembly 1114 may include a housing for the opticalelements of the optical system 1113. The housing of the actuatorassembly can be integrated into a housing of the HMD 1110. For example,the HMD 1110 may include a housing similar to the housing 220 in FIG. 2.The actuator assembly 1114 and the optical system 1113 may be positionedalong an optical path between the eye of the user and the display device1112, with the optical path passing through the optical elements ofoptical system 1113. A separate actuator assembly and optical system maybe provided for each eye of the user.

The proximity sensor 1116 can be a sensor configured to detect that theuser is wearing the HMD 1110. For example, the proximity sensor 1116 canbe a simple mechanical switch that is activated when the user's head ispressed against a frame of the HMD 1110. Alternatively, the proximitysensor 1016 can be a resistive or capacitive touch sensor configured todetect contact with the user's head based on electrical measurements. Insome embodiments, the proximity sensor 1116 is an optical sensor.

The illuminator 1118 is an electrically triggered light source thatgenerates light for use in connection with presentation of image contenton the display device. For example, the generated light could be used incombination with one or more optical sensors 1120 to perform eyetracking or tracking of head movements in order to update image contentfrom one or more applications executed by the control unit 1130. In someembodiments, the generated light may be used to perform a measurementthat determines a degree to which a vision of the user needs to becorrected, e.g., a degree of presbyopia or visual accommodation error.The illuminator 1118 can be placed in a frame of the HMD 1110 orintegrated into an optical component such as the display device 1112.

The optical sensor 1120 can be an image sensor configured to capture 2Dand/or 3D image data, for example, a 2D image of the user's eye or theexternal environment around the user.

The position sensor 1122 can be a gyroscope, an accelerometer, a globalpositioning system device, or any other device that detects changes inthe location and/or orientation of the HMD 1110.

The inertial measurement unit 1124 is an electronic device thatgenerates data indicating an estimated position of the HMD 1110, e.g.,based on measurement signals received from the position sensor 1122. Themeasurement signals can include, for example, signals indicative ofroll, pitch, yaw, or acceleration.

The I/O interface 1140 is a device that allows the user to send actionrequests to the control unit 1130. An action request is a request toperform a particular action. For example, an action request may be tostart or end an application or to perform a particular action within theapplication.

The control unit 1130 is configured to direct the operation of the HMD1110 including, for example, selecting image content for presentation onthe display device 1112, activating the illuminator 1118, and adjustingthe optical system 1113 (e.g., moving or shaping one or more opticalelements of the optical system 1113 using an embodiment of an actuatorassembly). The control unit 1130 includes an optical system adjustmentmodule 1132, a tracking module 1134, one or more processors 1136, acontrol information data store 1137, and an application store 1139. Thecontrol unit 1130 can include components that are integrated into theHMD 1110. In some embodiments, one or more components of the controlunit 1130 are remotely located. For example, the control informationdata store 1137 can be located on a remote server or distributed betweena memory of the control unit 1130 and a remote server.

The control unit 1130 outputs signals that control the actuator of theactuator assembly 1114. The control signals can be sent directly to theactuator (e.g., the control unit 1130 may output voltage or currentsignals that drive the actuator) or sent to a power source that producespower for the actuator (e.g., a voltage or current generator).

The control information data store 1137 stores control information thatthe control unit 1130 uses for controlling the actuator assembly 1114.For example, the control information may include a lookup table mappingone or more optical characteristics of the optical system 1113 to targetpositions of displacement elements in the actuator assembly 1114.

The application store 1139 stores one or more applications for executionby the control unit 1130. An application is a set of instructionsexecutable by a processor, for example instructions that cause theprocessor to generate content for presentation to the user on thedisplay device 1112. Examples of applications include: gamingapplications, conferencing applications, video playback application, orother suitable applications.

The optical system adjustment module 1132 can be implemented in hardwareand/or software and is configured to adjust one or more opticalcharacteristics of the optical system 1113 using the control informationstored in the control information data store 1137. In some embodiments,the optical system adjustment module 1132 comprises instructions storedon a non-transitory computer-readable medium, the instructions beingexecutable by the processor 1136 to control the actuator assembly 1114.

The tracking module 1134 can be implemented in hardware and/or softwareand is configured to track changes in the position of the HMD 1110and/or the position of the user's facial features (e.g., eye tracking).In some embodiments, the tracking module 1134 may track the movements ofthe HMD 1110 and correlate the HMD movements to movement of the user'shead.

The processor 1136 executes instructions from applications stored in theapplication store 1139 and/or instructions provided to the processor1136 by the optical system adjustment module 1132 or the tracking module1134. The processor 1136 can receive various items of information usedin the applications. This includes, for example, position information,acceleration information, velocity information, and captured images.Information received by processor 1136 may be processed to produceinstructions that determine content presented to the user on the displaydevice 1112.

The foregoing description of the embodiments of the disclosure has beenpresented for the purpose of illustration; it is not intended to beexhaustive or to limit the disclosure to the precise forms disclosed.Persons skilled in the relevant art can appreciate that manymodifications and variations are possible in light of the abovedisclosure.

Some portions of this description may describe the embodiments of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software and/or hardware.

Steps, operations, or processes described may be performed orimplemented with one or more hardware or software modules, alone or incombination with other devices. Although the steps, operations, orprocesses are described in sequence, it will be understood that in someembodiments the sequence order may differ from that which has beendescribed, for example with certain steps, operations, or processesbeing omitted or performed in parallel or concurrently. In someembodiments, a software module is implemented with a computer programproduct comprising a computer-readable medium containing computerprogram code, which can be executed by a computer processor forperforming any or all of the steps, operations, or processes described.

Embodiments of the disclosure may also relate to an apparatus forperforming the operations described. The apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the disclosure be limited not bythis detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the embodimentsis intended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. An actuator assembly, comprising: a plurality oflead screws; a mechanical linkage that intercouples the plurality ofleads screws, the mechanical linkage configured to simultaneously rotatethe plurality of lead screws; a plurality of displacement elementsincluding a first displacement element and a second displacementelement, wherein: each displacement element is configured to act upon arespective lens through translational motion of the displacement elementin response to rotation of the plurality of lead screws, the firstdisplacement element and the second displacement element are configuredto undergo translational motion together due to the first displacementelement and the second displacement element both being coupled to theplurality of lead screws, translational motion of the first displacementelement causes the first displacement element to act upon a first lensby applying force to a flexible membrane of the first lens, andtranslational motion of the second displacement element causes thesecond displacement element to act upon a second lens located behind thefirst lens by applying force to a flexible membrane of the second lens;a first drive nut located between the first displacement element and afirst lead screw of the plurality of lead screws, wherein a portion ofthe first drive nut is spaced apart from the first displacement elementto accommodate rotation of the first displacement element before sideloading of the first lead screw occurs; and a second drive nut locatedbetween the second displacement element and the first lead screw or asecond lead screw of the plurality of lead screws, wherein a portion ofthe second drive nut is spaced apart from the second displacementelement to accommodate rotation of the second displacement elementbefore side loading of the first lead screw or the second lead screwoccurs.
 2. The actuator assembly of claim 1, further comprising: anactuator configured to produce a rotational output that drives the firstlead screw of the plurality of lead screws.
 3. The actuator assembly ofclaim 1, wherein the first displacement element is a ring configured toapply force around a perimeter of the flexible membrane of the firstlens, and wherein the second displacement element is a ring configuredto apply force around a perimeter of the flexible membrane of the secondlens.
 4. The actuator assembly of claim 1, wherein the plurality of leadscrews have different threading.
 5. The actuator assembly of claim 1,wherein the mechanical linkage comprises a belt or cable, and whereinthe belt or cable wraps around each of the plurality of lead screws. 6.The actuator assembly of claim 1, wherein for each lead screw in theplurality of lead screws, the first displacement element and the seconddisplacement element are coupled to different sections along a shaft ofthe lead screw.
 7. The actuator assembly of claim 6, whereinsimultaneous rotation of the plurality of leads screws causes the firstdisplacement element and the second displacement element to move closertogether or farther apart, along the shaft of each lead screw.
 8. Theactuator assembly of claim 1, wherein: the flexible membrane of thefirst lens and the flexible membrane of the second lens form separatesealed cavities that each contains liquid, the first displacementelement is located outside the sealed cavity of the first lens, and thesecond displacement element is located outside the sealed cavity of thesecond lens.
 9. The actuator assembly of claim 1, wherein the firstdrive nut comprises a cylindrical section that is inner-threaded tomatch a threading of the first lead screw, the cylindrical section beinginserted through an opening of the first displacement element.
 10. Theactuator assembly of claim 9, wherein: the cylindrical section isseparated from the first displacement element by a first gap thataccommodates rotation of the first displacement element before sideloading of the first lead screw occurs, the first drive nut furthercomprises a flanged section that contacts a back side of the firstdisplacement element, and part of the flanged section is separated fromthe first displacement element by a second gap that accommodatesrotation of the first displacement element before side loading of thefirst lead screw occurs.
 11. The actuator assembly of claim 9, whereinthe first drive nut further comprises a flanged section that contacts aback side of the first displacement element, the flanged sectionincluding a flat portion that contacts an inner surface of a housingcontaining the actuator assembly such that rotation of the first drivenut in response to rotation of the first lead screw is prevented. 12.The actuator assembly of claim 1, wherein the second drive nut islocated between the second displacement element and the first leadscrew.
 13. A system, comprising: a head-mounted device including a firstlens and a second lens located behind the first lens; and an actuatorassembly housed within the head-mounted device, the actuator assemblyincluding: a plurality of lead screws; a mechanical linkage thatintercouples the plurality of leads screws, the mechanical linkageconfigured to simultaneously rotate the plurality of lead screws; aplurality of displacement elements including a first displacementelement and a second displacement element, wherein: each displacementelement is configured to act upon a respective lens throughtranslational motion of the displacement element in response to rotationof the plurality of lead screws, the first displacement element and thesecond displacement element are configured to undergo translationalmotion together due to the first displacement element and the seconddisplacement element both being coupled to the plurality of lead screws,translational motion of the first displacement element causes the firstdisplacement element to act upon the first lens by applying force to aflexible membrane of the first lens, and translational motion of thesecond displacement element causes the second displacement element toact upon the second lens by applying force to a flexible membrane of thesecond lens; a first drive nut located between the first displacementelement and a first lead screw of the plurality of lead screws, whereina portion of the first drive nut is spaced apart from the firstdisplacement element to accommodate rotation of the first displacementelement before side loading of the first lead screw occurs; and a seconddrive nut located between the second displacement element and the firstlead screw or a second lead screw of the plurality of lead screws,wherein a portion of the second drive nut is spaced apart from thesecond displacement element to accommodate rotation of the seconddisplacement element before side loading of the first lead screw or thesecond lead screw occurs.
 14. The system of claim 13, wherein the firstlens and the second lens are configured to direct light from an opticalaperture to an eye of a user wearing the head-mounted device.
 15. Thesystem of claim 14, wherein the plurality of lead screws are locatedalong a periphery of the optical aperture.
 16. The system of claim 14,wherein at least one of the first lens or the second lens isasymmetrically shaped.
 17. The system of claim 14, wherein the firstdisplacement element is a ring configured to apply force upon theflexible membrane of the first lens, and wherein the second displacementelement is a ring configured to apply force upon the flexible membraneof the second lens.
 18. The system of claim 13, wherein the plurality oflead screws have different threading.
 19. The system of claim 13,wherein for each lead screw in the plurality of lead screws, the firstdisplacement element and the second displacement element are coupled todifferent sections along a shaft of the lead screw.
 20. The system ofclaim 19 wherein simultaneous rotation of the plurality of leads screwscauses the first displacement element and the second displacementelement to move closer together or farther apart, along the shaft ofeach lead screw.